This invention generally relates to a drop-on-demand ink jet printer in which the flow of ink toward the ink reservoir during droplet ejection is controlled.
Drop-on-demand ink jet printers selectively eject droplets of ink toward a receiver to create an image. Such printers typically include a print head having an array of nozzles, each of which is supplied with ink from a reservoir. Each of the nozzles communicates with a chamber that can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle. Some such printers, commercial and theoretically-known, use piezoelectric transducers to create the momentary forces necessary to generate an ink droplet. A squeezing action by the piezoelectric transducers causes ink to flow out of the nozzles, but also causes some ink to flow backward toward the ink reservoir. Considerable energy is wasted, as not all of the pressure generated by the piezoelectric transducers results in droplet formation. Thus, a higher voltage must be applied to compensate for the loss.
The amount of backward flow of ink may be reducible by providing a narrow entry channel into the ink chamber from the reservoir. However, this would result in an undesirable increase in chamber refill time.
According to the present invention, the amount of backward flow of ink is reduced, while allowing free forward flow into the ink chamber by providing a valve in the entry channel to the ink chamber. During droplet ejection, the valve chokes back flow to improve efficiency. During chamber refill, the valve is opened, reducing refill time.
While any valve would be useful, response time of the valve should be better than the refill time for the chamber. According to a preferred embodiment of the present invention, a thermally activated valve, in which heat causes a thermal-reversible gel to form in the fluid channel, is provided to impede ink flow. When the heat is reduced, the gel returns to a freely-flowing fluid. By timing the heat pulse and the piezo device, drop ejection efficiency and refill time can be optimized.
According to one feature of the present invention, a drop-on-demand ink jet printing system includes a channel having a nozzle orifice through which ink droplets are ejected when ink in the channel is subjected to a momentary positive pressure wave. An ink feed passage opens into the ink channel to transport ink into the channel from an ink reservoir. A selectively-actuated valve, associated with the ink feed passage, restricts the flow of ink through the ink feed passage when actuated. The valve is actuated in timed association with the momentary pressure wave, whereby flow of ink past the valve from the ink channel towards the reservoir is inhibited.
According to another feature of the present invention, the ink feed passage is a microfluidic channel, and the selectively-actuated valve comprises a heater in thermal contact with at least a portion of the associated microfluidic channel. Thermally-responsive ink in the ink feed passage can selectively be heated by the heater such that the thermally-responsive ink will be caused to increase in viscosity to thereby restrict ink flow through the ink feed passage.